1. Field of the Invention
The present invention relates to systems for measuring biological functions of a patient and, more particularly, to systems which use a thermistor catheter to measure biological functions continuously. The present invention also relates to methods and apparatus for reducing noise in a signal provided by a thermistor catheter to a processing instrument so that the instrument may more accurately measure biological functions such as cardiac output. The present invention is particularly beneficial in environments in which large amounts of thermal and/or high-frequency noise are present.
2. Description of the Related Art
Biological functions of a patient, particularly cardiac functions such as intercardiac pressure, flow rate, and ejection fraction, are monitored by physicians in order to obtain vital information necessary to perform various cardiovascular procedures. Of the various cardiac functions, cardiac output indicates to a physician the amount or volume of blood pumped by the heart per unit time. Information on a patient's cardiac output indicates the effectiveness of the heart as a pump and of blood circulation.
Cardiac output may be measured continuously throughout a particular surgical procedure. The cardiac output of a patient may also be measured before and after a surgical procedure. To measure cardiac output, the physician inserts a catheter into the cardiovascular system and positions the catheter at a target sight. The catheter has a heating element which is activated to heat the blood flowing around the catheter at the target sight. A temperature sensor, for example, a thermistor, positioned downstream of the heating element provides a signal from which blood temperature is calculated. Cardiac output may then be calculated based upon the blood temperature in conjunction with other variables.
The signal from the temperature sensor is relatively small. Accordingly, conventional temperature-sensing catheters are vulnerable to a number of error-inducing sources. A surgical theater is a particularly noisy electrical environment, due in part to multiple machines operating simultaneously. The human body is also very noisy. All of this electronic, thermal, and physiological noise decreases the accuracy of the signal from the temperature sensor and, therefore, affects the accuracy of cardiac output measurements and hinders a physician's ability to successfully diagnose, monitor, and treat cardiovascular problems of a patient. Although sophisticated signal-processing techniques and equipment have been employed in the prior art to reduce the amount of noise (and resultant error) as much as possible, there still exists a significant amount of noise in the measurement of cardiac output.
Conventional signal-processing techniques are implemented either on the catheter itself or in the processing instrumentation. As catheters are discarded after a single use, it is not cost effective to include signal-processing circuitry on the catheter. Such implementation increases the cost of the catheter and is wasteful as the catheter and the circuitry are disposed of after one use. Alternatively, the circuitry may be incorporated in the processing instrumentation. However, there are thousands of existing instruments in use which become obsolete and need to be replaced in view of new instruments including signal-processing circuitry. This is also expensive and wasteful.
Errors may also be introduced into the measurement of cardiac output when a thermal time response of the thermistor is fast. The thermal time response of a thermistor indicates how quickly the thermistor reacts to changes in temperature. One method attempting to overcome introduced errors is to control the time response of the thermistor by adding or subtracting material on the catheter at the thermistor. The amount of material controls the thermal time response mechanically. More material results in lower thermal diffusion, and less material results in higher thermal diffusion. One of the drawbacks of this approach is that it is expensive, particularly on disposable catheters. In addition, it is difficult to control the time response consistently with this mechanical approach.